The Allegheny County Health Department issued its second combined sewer overflow (CSO) alert of the season Thursday.

The advisories notify swimmers, boaters and other river-goers if the water has been contaminated by raw sewage after heavy rainfall clogs waste treatment facilities. The length of the advisories depends on the time it takes for the sewer systems to return to normal levels.

The CSO alerts do not prohibit recreational river activity, but advise the public to reduce water contact, especially those with weak immune systems or open cuts and sores.

Ronald Voorhees, acting director of the Allegheny County Health Department, said the alerts are used to warn the public of potential bacterial dangers in the water.

“Swimming and high contact activities in the rivers – we recommend minimizing that contact just to reduce the chance of getting a disease from the sewer overflow,” Voorhees said.

Voorhees said he doesn’t expect anyone to contract diseases like hepatitis, but serious infections and diarrhea could result from excessive contact with the contaminated water.

There are 30 sites along the Allegheny, Monongahela, Ohio and Youghiogheny rivers, including boating docks and marinas that are putting up flags to notify the public when an alert is in effect. The orange flags will be flown with the letters “CSO” in black. Information on alerts can also be found on the Allegheny County Health Department website.

Voorhees said the best way to stay safe is to use common sense.

“Certainly, I recommend not swallowing river water in any event,” Voorhees said. “Humans are not the only source of pathogens in an open body of water like that. So I think there are good precautions to take.”

The frequency and duration of the alerts have varied year-to-year since first being used in 1995. Last summer saw eight alerts lasting an average of four days.